cmd/compile/internal/gc: remove binary package export format

This CL removes all unused code from bimport.go and bexport.go.

In the interest of keeping this CL strictly delete-only and easier to
review, the task of consolidating the vestigial code elsewhere is left
to future CLs.

Change-Id: Ib757cc27e3fe814cbf534776d026e4d4cddfc6db
Reviewed-on: https://go-review.googlesource.com/c/139338


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>